Acoustic Doppler Current Profilers, or ADCPs, were first introduced and used by oceanographers in the late 1970s. ADCPs use acoustic beams to measure ocean current velocity over vertical profiles with ranges of a few meters to around 1000 m. They transmit sound, receive the echoes and process the echoes to detect changes in frequency associated with Doppler shifts produced by the relative velocity of the water and the ADCP. The result is a velocity profile, or velocity measurements in a series of depth cells forming a profile over depth. The ADCP either records the velocity profile data on an internal recorder or transmits the data to an external device or computer.
Starting in the 1970s, ADCPs initiated a revolutionary change for people who studied the ocean. A single ADCP could replace the velocity measurements of a string of current meters while producing data that was both better and easier to use. Current meters were spaced irregularly, they left gaps when they failed, and the drift of their internal clocks produced data that was difficult to synchronize. In contrast, ADCPs produced data with uniform and regular coverage, both over depth and in time. An ADCP's data at each depth are perfectly synchronized with the data from all the other depths. As ADCPs replaced strings of moored current meters, they also made it possible to survey currents from moving ships. Today, most of the current data collected in the ocean is collected by some variation of an ADCP.
ADCPs transmit sound into the ocean through acoustic transducers, which serve as well to receive the echoes returning to the ADCP. Sound is well suited for making remote measurements of current velocity in the ocean because sound propagates over much larger distances in the ocean than light or other electromagnetic radiation. ADCPs use electronics to create the acoustic signals transmitted by the transducers, process the received echo signals, and compute velocity.
Transducers are carefully designed to produce narrow, directional acoustic beams for which the beam directions, relative to the ADCP are known precisely. Each beam measures a single component of velocity, the velocity component parallel to the beam. The beam must be highly directional in order to accurately measure its component. Since a velocity vector has multiple components, velocity measurement requires multiple beams. Therefore, commercial ADCPs use a set of beams, all pointing in different directions to measure velocity.
Most ADCPs use individual piston transducers, each emitting a corresponding acoustic beam. They are called piston transducers because they are made from cylindrical disks of piezoelectric ceramics, which vibrate in “piston mode” to produce a single beam of sound along the axis of the disk. The surface of a piston transducer that faces the water also faces in the direction the beam points.
Piston transducers are normally combined by the manufacturer into transducer assemblies, which hold several transducers, each precisely aligned relative to one another. Precise alignment is necessary for the computation of the velocity vector from the single components measured by the beams.
Some ADCPs use phased arrays, which are a different sort of transducer assembly. Phased arrays include a large number of small transducer elements, and complex electronics. They use complex signal processing involving time delays or phase lags to produce multiple beams from a single aperture. Phased arrays and piston transducer assemblies have strengths and weaknesses relative to each other in terms of the quality of the results they produce, but their velocity profiles are otherwise the same.
A transducer assembly built using piston transducers forms a set of beams using one transducer to form each beam. A single phased array transducer can also form a set of beams. The ADCP uses the velocity components measured with a set of beams to compute a velocity profile. Whereas each velocity component is the velocity parallel to a single beam, the computed velocity is a vector with both magnitude and direction.
Velocity measurement in the ocean depends on the fact that ocean currents usually are both horizontal and horizontally homogeneous. Currents vary rapidly with depth, but they vary slowly over horizontal displacements. With the use of a compass and tilt sensor, two beams are sufficient to measure the velocity as long as the vertical velocity is small (which normally is a reasonable assumption in the ocean). However, nearly all commercial ADCPs use beam sets having three or four slanted beams, which enables them to measure both horizontal and vertical components of the velocity. The use of more than two beams also improves the accuracy of the velocity measurements.
Measurement of horizontal velocity in vertical profiles requires that beams be slanted relative to vertical. Purely vertical beams can perform useful functions such as measuring the distance to the surface, but they do not provide information about the horizontal velocity. ADCP beam sets are characterized as having a characteristic slant angle, most often in the range 20-30 degrees relative to vertical. A beam with a 90-degree slant angle is slanted horizontally. The slant angle refers to the angle between the beam and vertical. Each beam in a beam set typically has its own direction component in the horizontal plane, but all of the beams in a typical beam set have the same slant angle. Therefore, beam sets normally have a single characteristic slant angle.
When beams are created by piston transducers, each transducer set produces a corresponding beam set. The beam sets are physically separated and occupy different space; they may also have different slant angles. Phased arrays can also produce multiple beam sets, which can differ from one another by the slant angle or by occupying different physical space.
Beams, beam sets and transducers can all face upwards or downwards depending on the slant angles. A beam that points above the ADCP is said to face upward. If all of the beams in a beam set point above the ADCP, then the entire beam set faces upward. A transducer assembly emitting these beams would also be said to face upward. The slant angle of an upward facing beam set has the opposite sign from the slant angle of a downward facing beam set even when the magnitude is the same. There is no general convention whether slant angles should be defined as positive upward and negative downward, or visa versa, but ADCPs nevertheless account for this angle in their processing.
Slanted beams produce a tension between the need to obtain data over a profile and the need to measure velocity accurately. A purely horizontal beam provides the most accurate measurement of horizontal velocities, and the velocity uncertainty increases as the beam becomes more vertical. While purely vertical beams provide no information about horizontal currents, they give the greatest possible vertical profiling range; the useful vertical range decreases as a beam becomes more horizontal. The typical 20-30 degree beam angles represent a compromise of these conflicting needs. It is worth noting that while the beam angle is fixed relative to the ADCP, the beams can all vary in angle relative to the earth. For example, an ADCP mounted on a mooring can tilt back and forth, which changes the beam angle relative to the earth as the ADCP moves about. However, as long as the tilting is not too large, the ADCP tilt has a relatively unimportant effect on the measurement quality and can be ignored.
Compromises associated with the beam angle provide one example of the many trade-offs involved in the design and use of ADCPs. Another compromise involves the acoustic frequency. The frequencies commonly used in ADCPs range from around 30 kHz up to around 2.5 MHz. Low frequency sound propagates further than high frequency sound, so the lowest frequencies are used for the greatest profiling ranges. On the other hand, higher frequency sound produces velocity measurements with lower velocity uncertainty, and it enables measurement with smaller depth cells.
Transducer assemblies can be built with transducers having different frequencies. In some circumstances, the data from beams having one frequency can be used to improve the profile results from the beams with the second frequency. If two frequencies are used for the sole purpose of measuring a single velocity profile, the beams of the two frequencies act like a single beam set. However, if the two frequencies produced independent velocity profiles, they clearly produce two independent beam sets.
ADCP users commonly swap an ADCP's transducer assembly to change the ADCP's beam set. Swapping the transducer allows the user to change the locations and/or angles of the individual beams, but not normally the frequency since the frequency is usually fixed by the electronics. ADCPs built using piston transducers traditionally produce a single beam set with each transducer assembly.
Another trade-off involves the acoustic power. An ADCP that uses higher power can measure velocity further from the ADCP, but it will then deplete its battery more quickly, therefore shortening the duration of the ADCP's deployment. There are limits to the acoustic power that a transducer can actually get into the water, and the limits vary with frequency.
Another trade-off involves the mode of operation, which the ADCP uses to collect data. Typical ADCPs are able to adjust a wide variety of parameters that affect how they collect data, each of which incurs trade-offs. Examples include the transmit pulse length, the depth cell size, the ping repetition rate, the time interval between which average measurements are made and other details of how the pulse is transmitted. All of these parameters could be made to be user adjustable, and all fall into the same category under the mode of operation.
The transmit pulse length and depth cell size are related and they are often set to the same values. Both are measured in terms of a duration (milliseconds). With scaling by the speed of sound in water, the duration can be expressed as a distance (meters) corresponding to the size of the depth cell. A velocity profile is a set of velocity measurements in a sequence of depth cells. Lengthening the transmit pulse enables the ADCP to get more energy into the water, which enables greater range, but at the expense of battery life. Longer transmit pulses reduce the velocity uncertainty, but also increase the size of depth cells.
In simple terms, the velocity profiling range is the product of the number of cells and the cell size (plus an offset near the transducer). In practice, the range is limited by a number of factors, the most important of which is the acoustic frequency. As the distance from the ADCP increases, the acoustic signal/noise ratio falls to the point where the velocity data becomes too noisy to use. ADCPs are able to compute and record velocity from beyond this range, but the data are not useful. Users can optimize data collection for long range, for example, by maximizing power, using long transmit pulses, long depth cells, and large blanking, but maximizing the profiling range can also impair the ability of the ADCP to measure velocity close to the ADCP.
ADCPs typically produce velocity measurements that are the average of many pings. The time between the pings and the number of pings per measurement determine the measurement interval. Each ping includes transmitting an acoustic pulse, receiving the echo and computing velocity in the depth cells from the echo. The velocity measurement obtained from a single ping is typically too noisy to use by itself, but the average of a number of these pings is less noisy and therefore more useful. It is worth noting that some pings actually can be divided into sub-pings in which a sequence of transmission pulses work together to produce a complete profile. For example, one sub-ping can provide a coarse measurement while the next sub-ping provides a fine measurement that is somehow constrained by the first sub-ping. In this way, a sequence of pulses produces a single velocity estimate, and this sequence of pulses can be considered to form a single ping. Another example is an ADCP's bottom-track pulses. Bottom track pulses provide an earth-reference velocity, which converts the current profile measurement from ADCP-referenced to earth-referenced. The bottom-track pulse improves the quality of the data in a current profile, but it does not change the profile's temporal, spatial or velocity scales.
In order to obtain a better velocity estimate, the ADCP can reduce its ping interval (within limits) to obtain more data in a given measurement interval. Increasing the number of pings averaged in a given interval makes the velocity measurement more accurate, but at the cost of depleting the battery quicker. Alternatively, an ADCP can average more pings by waiting longer before it computes the next average. In this way, increasing the measurement interval makes the result more accurate.
Another way in which ADCPs can change the mode of measurement is to vary the phase or amplitude of the transmit pulse while it is being transmitted. There are many methods available for implementing such variations, each with its corresponding means of processing. These methods also involve trade-offs including measurement uncertainty, depth cell size, measurement interval, and profiling range.
It is worth noting that some of the trade-offs involve parameters that can be adjusted within a given instrument, while other parameters are fixed. For example, most of today's ADCPs fix both the beam angles and the frequency (though some ADCPs with phased arrays could provide some control over the beam angle). Some ADCPs have fixed transmit power while others allow users to control the transmit power. Most ADCPs allow users to vary most of the other parameters discussed above.
The trade-offs discussed thus far are important because they affect the ability of the ADCP to measure specific ocean processes. People study a wide variety of ocean processes for a wide variety of purposes. Examples of ocean processes include ocean circulation currents like the Gulf Stream, coastal currents, internal waves, near-surface wind driven currents, Langmuir currents, currents in bottom boundary layers, turbulence, and the orbital velocities produced by surface waves. Each process has its own characteristic scales: typical velocities, typical time and length scales of variation. Scientists use the Navier-Stokes equations to model these processes, but the equation is too difficult to solve in general, so each process has its own particular simplifications and approximations that enable scientists to model and understand the process. Ocean processes that are observable to an ADCP include more than just ocean physics. Engineers often study physical processes with an eye toward learning about the forces that affect operations in the ocean or the survivability of ocean structures. Such engineers are concerned about the processes in which ocean current velocity affects offshore operations and structures.
Users decide which profiling parameters to use after considering the ocean process they plan to study. For example, an oceanographer observing the Gulf Stream will likely require a relatively long profiling range but would be satisfied with large depth cells and long intervals between measurements. A person studying the boundary layer under the Gulf Stream (i.e. the near-bottom flow that is affected by the friction of the bottom) will study a smaller profiling range, but will typically look for smaller depth cells and shorter measurement intervals. Engineers will adjust their measurements according to the structures they are working with. For example, an engineer placing a floating drilling rig in the Gulf Stream may not need to profile as deeply as one designing a production platform. On the other hand, the one concerned about the drill rig may need shorter intervals between his measurements.
A need exists for ADCPs that study multiple ocean processes, including the relationships among these processes. A further need exists to use a single instrument to study multiple processes in place of two or more instruments. The need also exists for a single instrument that can study multiple processes that occupy different nearby physical spaces.
Where people deploy two ADCPs nearby one another, there is a need for synchronizing the two data sets. If the data collected by two ADCPs could be coordinated into a single ADCP, this problem would be solved, but then there is a need to allow the two data sets to be optimized separately according to what the ADCPs are deployed to observe. For example, a scientist studying the Gulf Stream would observe the bottom boundary layer under the Gulf Stream with a different ADCP because the scales are so different, but because the two ADCPs' clocks drift relative to one another, he would struggle when trying to compare the two data sets.
Today's commercial ADCPs are designed so that each transducer beam, its corresponding signal path, and the processes that control the operations along each path are all identical. A single transmit circuit serves all of its channels. A single timing controller with parallel lines to each signal path controls all of them the same. The profiling parameters set by the ADCP user translate into complex and precise timing sequences that control how the ADCP works, and the design of an ADCP is simplified by applying the same timing to all of the ADCP's channels. This approach has generally worked well for both manufacturers and users because the simplifications reduce the cost of ADCPs, simplify their use and produce high quality results that users appreciate.